If you download from the RefSeq ftp assemblies site, you don't need to concatenate contigs / scaffolds, there is a single file for the assembly. For example, for the assembly you provided as example, the file in question is:
In addition to the whole assembly, the ftp folder has files with protein translations, gene annotations, and so forth.
Just in case you already downloaded the files, or if there is no single file with the whole assembly, you can concatenate several fasta into one with cat:
cat NZ_LNUL01000*.fas > NZ_GG668845.1.fas